1. Technical Field
The present invention relates to spindle motors employing dynamic-pressure bearings in which oil is the working fluid, and to disk drives equipped with such spindle motors. The invention relates in particular to miniature, low-profile spindle motors that drive recording disk 2.5 inches and under, and to disk-drives equipped with such spindle motors.
2. Description of the Related Art
Dynamic-pressure bearings in which the fluid pressure of a lubricating fluid such as oil interposed in between the shaft and the sleeve is exploited in order to support the two letting the one rotate against the other have been proposed to date as bearings for spindle motors employed in disk drives that drive hard disk and like recording disks.
FIG. 1 depicts one example of a spindle motor employing dynamic-pressure bearings. This spindle motor in which conventional dynamic pressure bearings are employed is configured with a pair of axially separated radial bearing sections d, d in between the circumferential surface of the motor shaft b, which is integral with the rotor a, and the inner peripheral surface of the motor sleeve c, into which the shaft b is rotatively inserted. Likewise, a pair of thrust bearing sections g, g is configured in between the upper surface of a disk-shaped thrust plate e that projects radially outward from the circumferential surface of the shaft b on one of its ends, and the flat surface of a step formed in the sleeve c, as well as in between the lower surface of the thrust plate e and a thrust bush f that closes off one of the openings in the sleeve c.
A series of micro-gaps is formed in between the shaft b and thrust plate e, and the sleeve c and thrust bush f, and oil as a lubricating fluid is retained continuously without interruption within these micro-gaps. The oil retained in the micro-gaps is exposed to the air only within a taperseal area h provided at the upper-end opening (the other opening in the sleeve c) of the gap formed in between the circumferential surface of the shaft b and the inner peripheral surface of the sleeve c. (This sort of oil-retaining structure will be denoted a “full-fill structure” hereinafter.) The dynamic-pressure bearings further include herring-bone grooves d1, d1 and g1, g1 that are linked pairs of spiral striations formed in the radial bearing sections d, d and thrust bearing sections g, g. In response to the rotor a rotating the grooves d1, d1 and g1, g1 generate maximum dynamic pressure in the bearing-section central areas where the spiral striation links are located, thereby supporting loads that act on the rotor a.
With dynamic-pressure bearings in a full-fill structure, when the rotor a begins rotating, pumping by the dynamic-pressure-generating grooves d1, d1 and g1, g1 acts to draw oil in toward the center areas of each of the radial bearing sections d, d and thrust bearing sections g, g, peaking the fluid dynamic pressure in the bearing center areas; but the downside of this is that along the bearing edge areas the oil internal pressure drops. In particular, in response to the pumping by the dynamic-pressure-generating grooves d1, d1 and g1, g1 the internal pressure of the oil drops-falling below atmospheric pressure and eventually becoming negative-in the region between the pair of radial bearing sections d, d among where oil is retained between the circumferential surface of the shaft b and the inner peripheral surface of the sleeve c, and in the region adjacent the outer periphery of the thrust plate e located in between the thrust bearings g, g among where oil is retained surrounding the thrust plate e.
If negative pressure within the oil has been brought about, during such operations as when the bearings are being charged with oil for example, air will dissolve into the oil and appear in the form of bubbles. Sooner or later the bubbles will swell in volume due to elevations in temperature, causing the oil to exude outside the bearings. This leakage impairs the endurance and reliability of the spindle motor. Negative pressure occurring within the oil can also lead to the dynamic-pressure-generating grooves coming into contact with air bubbles, which invites vibration incidents and deterioration due to NRRO (non-repeatable run-out). Such consequences impair the rotational precision of the spindle motor.
If because some factor is off in the manufacturing process the radial clearance dimension of the micro-gap formed in between the inner peripheral surface of the sleeve and the circumferential surface of the shaft is formed wider at the lower end axially than at the upper end, then an imbalance in the pumping by the dynamic-pressure-generating grooves d1, d1 in the radial bearing sections d, d will arise, and in the oil retained in between the inner peripheral surface of the sleeve and the circumferential surface of the shaft the pressure on the upper end axially will become higher than the pressure on the lower end axially. Pressure is consequently transmitted from along the upper axial end to along the lower axial end of the microgap formed in between the inner peripheral surface of the sleeve and the circumferential surface of the shaft, raising higher than is necessary the internal pressure of the oil retained in between the thrust-plate undersurface and the thrust bush and producing over-lift on the rotor, lifting it more than the predetermined amount.
Incidents of over-lift on the rotor give rise to frictional wear on the thrust plate and the sleeve by bringing them into contact; frictional wear is one factor to blame for spoiling bearing endurance and reliability. What is more, in the case of spindle motors for driving hard disk, as a consequence of the scaling-up of hard disk capacity, hard-disk recording faces and magnetic heads are being arranged extremely close to each other, and thus over-lift on the rotor brings the hard disk and magnetic heads into contact, leading to disk crash.
In addition, when the rotor a begins rotating, pumping by the dynamic-pressure-generating grooves d1, d1 and g1, g1 acts to draw oil in toward the center areas of each of the radial bearing sections d, d and thrust bearing sections g, g, peaking the fluid dynamic pressure in the bearing center areas. Nevertheless, when the motor rotates at low speed, the load bearing pressure generated by the bearing sections are insufficient. In particular, low thrust load bearing pressure gives rise to frictional wear on the thrust plate e and the sleeve c or the thrust bush f by bringing them into contact; frictional wear is one factor to blame for spoiling bearing endurance and reliability.
Further, with the advent of the application of disk drives in miniature devices such as portable information terminals, demands are on the rise to make the spindle motors used in the disk drives even miniature and slimmer. If the axial height and diameter of the motor is reduced, the space for containing stator and rotor magnet is limited and then sufficient driving force or torque cannot be obtained. As the result, when the motor starts to rotate, the driving force or torque of the motor becomes lower than the frictional torque and/or loss caused by being in contact with the thrust plate e and the sleeve c or the thrust bush f and the motor can not rotate.